A preferred task in Rs3 will give you double the chances of getting that monster as your slayer task.
The way it works is quite simple. Normally you get 1 “roll” on the slayer monster table (similar to how item drop tables work in runescape). With the preferred task set you will get an extra roll and if the monster on your preferred list comes up then you get assigned that as your task otherwise it will default back to the first roll on the table.
If you are using VIP slayer tickets then you’ll have even more chances of rolling a creature that you want on your preferred list.
Generally it is recommended to fill up your block list before your preferred list as that will block 100% of tasks you don’t want compared to roughly 6% increased chance at getting a task you want.
It’s far better to get rid of a really annoying task like black demons, greater demons or something like dust devils. Whatever gives you the most pain I would recommend blocking.